Piezoelectric component and method of manufacturing same

ABSTRACT

A piezoelectric component includes a substantially rectangular shaped piezoelectric element, a first elastic material covering at least a pair of end portions of the piezoelectric element, the pair of end portions including edge parts of the piezoelectric element, a second elastic material covering the entire piezoelectric element and the first elastic material, and an outer-cladding resin covering the whole circumference of the piezoelectric element which is covered with the second elastic material.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a piezoelectric component and amethod of manufacturing the piezoelectric component. In thepiezoelectric component, a periphery of a piezoelectric element iscovered by an elastic material, and the circumference of the elasticmaterial and the piezoelectric element is covered by an outer-claddingresin.

[0003] 2. Description of the Related Art

[0004] Japanese Unexamined Patent Publication No. 1-228310 describes apiezoelectric component in which a periphery of a piezoelectric elementhaving end electrodes connected to lead terminals is covered by anelastic material such as a silicone rubber, and the circumferencethereof is sealed by an outer-cladding resin.

[0005] In order to manufacture such a piezoelectric component, first, alead terminal is attached to a piezoelectric element. Then, a siliconerubber layer is provided on the piezoelectric element by the dipping thepiezoelectric element in a liquid-state silicone rubber and hardeningthe rubber layer by applying heat. Further, an epoxy resin layer isprovided on the exterior of the silicone rubber via a dipping method.Then, the epoxy resin is hardened by heat to obtain an outer-claddingresin.

[0006] The epoxy resin used for forming the outer-cladding resin layercontracts when it is hardened by heat, and thereby a stress caused bythe outer-cladding resin which contracts and compresses the interior ofthe piezoelectric component is generated. If the amount of stress whichcontracts and compresses the piezoelectric element in the piezoelectriccomponent changes, the oscillation characteristics thereof change. Thesilicone rubber is used because it is able to prevent the change of theoscillation characteristics of the piezoelectric element, and has adamping effect which suppresses the waveform distortion of thepiezoelectric element.

[0007] To reduce the stress generated in the piezoelectric element bythe outer-cladding resin, preferably, the difference between the edgeportion of the piezoelectric element and the outer-cladding resin i.e.,the minimum thickness of the silicone rubber, is larger than thecontraction amount of the outer-cladding resin when it is hardened, andfurther, is larger than the difference between the thermal expansions ofthe outer-cladding resin and of the piezoelectric element.

[0008] However, the dipping method for forming the liquid-state siliconerubber was not satisfactory for obtaining a required thickness of thesilicone rubber at the end portions of the piezoelectric element,because of the surface tension of the silicone rubber. Therefore, thetemperature characteristics of the piezoelectric components are largelyvaried by the stress generated in the piezoelectric element by theouter-cladding resin.

SUMMARY OF THE INVENTION

[0009] To overcome the above described problems, preferred embodimentsof the present invention provide a piezoelectric component and a methodof manufacturing the piezoelectric component, which greatly reduces thestress generated in the piezoelectric element by the outer-claddingresin while the function of damping the piezoelectric element ismaintained.

[0010] One preferred embodiment of the present invention provides apiezoelectric component, including a substantially rectangular shapedpiezoelectric element, a first elastic material covering at least a pairof end portions of the piezoelectric element, the pair of end portionsincluding edge parts of the piezoelectric element, a second elasticmaterial covering the entire piezoelectric element and the first elasticmaterial, and an outer-cladding resin covering the whole circumferenceof the piezoelectric element covered with the second elastic material.

[0011] In the above described piezoelectric component, the end portionsof the piezoelectric element, where it is most difficult to obtain asufficient thickness of an elastic material, is covered by the firstelastic material. Further, the periphery of the first elastic materialis covered by the second elastic material. That is, two elasticmaterials are provided at the edge parts of the piezoelectric element,where the piezoelectric element is most easily affected by thecontraction stress imparted by the outer-cladding resin. Therefore, evenwhen the contraction stress imparted by the outer-cladding resin variesaccording to temperature changes, a change in oscillationcharacteristics caused by the change of temperature is prevented becausethe first and second elastic materials having sufficient thicknessabsorb any changes of stress. Further, the damping effect is maintainedby the second elastic material to thereby prevent a waveform distortionor other harmful effects from being generated.

[0012] The modulus of elasticity and the hardness of the first andsecond elastic materials are determined in accordance with respectiverequired characteristics. In the case of the second elastic material.For example, it is preferable to set the hardness equal to or less than28 in “Shore Hardness A.”

[0013] Another preferred embodiment of the present invention provides amethod of manufacturing a piezoelectric component, including the stepsof forming an unhardened first elastic material partially on at least apair of end portions of a piezoelectric element, the pair of endportions including edge parts of the piezoelectric element, thenhardening the first elastic material, forming an unhardened secondelastic material on the entire circumference of the piezoelectricelement and the first elastic material, then hardening the secondelastic material and forming an unhardened outer-cladding resin on theentire circumference of the second elastic material covering thepiezoelectric element and the first elastic material, then hardening theouter-cladding resin.

[0014] By the above described method, the piezoelectric componentaccording to preferred embodiments of the present invention can bemanufactured easily.

[0015] For forming the unhardened first and second elastic materials,not only a dipping method but also using a soldering iron, dispenser, orother suitable method can be utilized. Similarly, a dipping method orother methods can be utilized for forming the unhardened outer-claddingresin.

[0016] Although the first elastic material and the second elasticmaterial may be the same, it is preferable to make the thixotropic index(call it a thixo index hereafter) of the first elastic material largerthan the thixo index of the second elastic material. The thixotropicproperty is one of the characteristics of fluid, and is defined as aproperty in which a viscosity varies nonlinearly depending on a shearingstress. It is easy to achieve sufficient thickness of the elasticmaterial when forming the elastic material on the piezoelectric elementif the thixo index of the elastic material is large. Therefore, if thethixo index of the first unhardened elastic material is made larger thanthe thixo index of the second unhardened elastic material, sufficientfilm thickness at the both edge parts of the piezoelectric element canbe obtained. On the other hand, there is no problem even when the thixoindex of the second elastic material is small, because it is easy tocoat the center portion of the piezoelectric element using the secondelastic material, and this construction is hardly affected by heat.

[0017] Preferably, the thixo index of the first and second elasticmaterial is preferably larger than about 1.7. If the thixo index isequal to or less than about 1.7, it is hard to achieve a sufficientthickness of the elastic material at the end portions of thepiezoelectric element because of a surface tension of the elasticmaterial.

[0018] Other features and advantages of the present invention willbecome apparent from the following description of the invention whichrefers to the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

[0019]FIG. 1 is a longitudinal sectional view of one preferredembodiment of a piezoelectric component according to the presentinvention.

[0020]FIG. 2 is a sectional view of FIG. 1 taken along the line II-II inFIG. 1.

[0021]FIG. 3 is a front view of the piezoelectric element shown in FIG.1 with which lead terminals are fixed.

[0022]FIG. 4 is a back view of the piezoelectric element shown in FIG. 1with which lead terminals are fixed.

[0023]FIG. 5 is a front view of the piezoelectric element with the leadterminals shown in FIG. 3, wherein a first elastic material is furtherprovided at the edge parts of the piezoelectric element.

[0024]FIG. 6 is a front view of the piezoelectric element with the leadterminals and the first elastic material shown in FIG. 5, wherein asecond elastic material is further provided on the entire surface of thepiezoelectric element and the first elastic material.

[0025]FIG. 7 is a graph showing the relationship between a thixotropicindex and a thickness of the elastic material at the edge portion of thepiezoelectric element.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0026] FIGS. 1 to 6 shows a three terminal type piezoelectric trapcomponent which is one preferred embodiment of a piezoelectric componentaccording to the present invention. This piezoelectric trap componentpreferably includes a substantially rectangular shapedpiezoelectric-ceramic substrate 1 which has a pair of substantiallyrectangular major surfaces. The substrate 1 is polarized along thelongitudinal direction of the major surfaces thereof. As shown in FIG. 3and FIG. 4, the divided electrodes 2 and 3 are disposed on both endportions of a first of the major surfaces of the piezoelectric-ceramicsubstrate 1. The ground electrode 4 is disposed at the approximatecenter portion of a second major surface of the piezoelectric-ceramicsubstrate 1. The divided electrodes 2 and 3 and the ground electrode 4are partially overlapping each other with the piezoelectric-ceramicsubstrate 1 disposed therebetween. A thickness shear vibration isgenerated at the opposite portion. The piezoelectric element of apreferred embodiment of the present invention is preferably constitutedby the piezoelectric-ceramic substrate 1 provided with the abovedescribed electrodes 2-4. Lead terminals 5 and 6 are respectively fixedand connected to the divided electrodes 2 and 3 via soldering or othersuitable joining process or member. A lead terminal 7 is fixed andconnected to the ground electrode 4. The lead terminals 5-7 arepreferably defined by plate-shaped or wire-shaped metallic material.

[0027] Edge portions of the piezoelectric-ceramic substrate 1 arerespectively covered by silicone rubber layers 10 and 11 which definethe first elastic material 1. The thixo indexes of the silicone rubber10 and 11 in an unhardened state are preferably larger than about 1.7 toachieve sufficient thickness of the first elastic material at the edgeportions of the piezoelectric-ceramic substrate 1.

[0028] The entire periphery of the piezoelectric-ceramic substrate 1with the silicone rubber layers 10 and 11 is preferably covered with asilicone rubber 12 which defines a second elastic material.

[0029] Since this silicone rubber 12 covers the vibrating portion of thepiezoelectric element, it is necessary to select a material property inconsideration of the piezoelectric property in addition to thethixotropic property in an unhardened state. In this preferredembodiment, the thixo index of the second silicone rubber 12 ispreferably larger than about 1.7, although it is preferably smaller thanthe thixo index of the silicone rubber layers 10 and 11. Moreover, inthe case of a piezoelectric trap component, in order to prevent thedeterioration of the piezoelectric property, it is preferable to makethe hardness of the second silicone rubber 12 equal to or less thanabout 28 in “Shore Hardness A”.

[0030] It is noted that the thixo index used here means a ratio of aviscosity at about 6 rpm and a viscosity at about 60 rpm i.e.,

the thixo index (Ti)=a viscosity at about 6 rpm/ a viscosity at about 60rpm.

[0031] The entire periphery of the piezoelectric-ceramic substrate 1with the silicone rubber 12 is covered via an outer-cladding resin 13.In order to secure an electric insulation, mounting strength, and otherdesired characteristics, an epoxy resin is preferably used for definingthe outer-cladding resin 13, for example.

[0032] A preferred embodiment of a method of manufacturing the abovedescribed piezoelectric trap is explained below. First, a piezoelectricelement 1 as shown in FIG. 3 and FIG. 4 is prepared. That is, the leadterminals 5-7 are respectively fixed and connected to the electrodes 2,3, and 4 respectively disposed on a pair of major surfaces of thepiezoelectric-ceramic substrate 1. Next, as shown in FIG. 5, the firstsilicone rubber 10 and 11 in a fluid state i.e., an unhardened state, ispartially provided so that the end portions of the piezoelectric element1, especially the edge portions of the piezoelectric element 1 arecovered. More specifically, the silicone rubber 10 and 11 in the fluidstate is provided on the edge portions of the piezoelectric element 1via iron, etc., then the silicone rubber layers 10 and 11 are hardened.

[0033] Next, as shown in FIG. 6, a second silicone rubber 12 in a fluidstate is provided on the entire surface of the piezoelectric element 1and then the second silicone rubber 12 is hardened. This process isperformed preferably by dipping the piezoelectric element 1 into thesecond silicone rubber 12 in the fluid state i.e., an unhardened state.To facilitate the dipping process, the second silicone rubber 12 may bediluted with xylene, or other suitable material or substance.

[0034] Next, the piezoelectric element I including the second siliconerubber 12 is dipped into an outer-cladding resin 13 in a fluid statei.e., an unhardened state. Then, the outer-cladding resin 13 coveringthe piezoelectric element 1 and the second silicone rubber 12 ishardened via heating. As a result, the piezoelectric trap shown in FIGS.1 and 2 is obtained.

[0035] For obtaining a stable piezoelectric property, preferably, thetotal thickness of the silicone rubber layers 10, 11 and 12 at the edgeportion 1 a of the piezoelectric element 1 is larger than the differencebetween the maximum contraction amount of the outer-cladding resin 13and the maximum contraction amount of the piezoelectric element 1. Thepreferable thickness can be easily obtained by forming the siliconerubber layers 10, 11 and 12 as described above.

[0036] Table 1 shows the relationship between a thixotropic index Ti ofa silicone rubber in a fluid state and a mean thickness of the siliconerubber layer at the edge portion of a piezoelectric element.

[0037] As apparent from Table 1, the thixotropic index Ti is preferablylarger than about 1.7 in order to achieve a predetermined thickness. Ifthe thixotropic index Ti is about 1.7 or less, the silicone rubber cannot be disposed at the edge portion of a piezoelectric element. TABLE 1Mean thickness of Silicone rubber at the Thixotropic index Ti edgeportion of a piezoelectric element 3.1 54.1 2.8 52.9 2.5 50.8 2.3 42.72.1 29.5 1.8  8.0 1.7 0  

[0038]FIG. 7 is a graph showing the relationship indicated in Table 1.

[0039] Table 2 shows the relationship between a thickness of a siliconerubber at the edge portion of a piezoelectric element and a Fotemperature characteristic of a piezoelectric trap component. The Fotemperature characteristic is determined by measuring the Fo temperatureof the piezoelectric trap component within a range of about −20 degreescentigrade and about +85 degrees centigrade and then calculating thechanging ratio based on Fo temperature at about +20 degrees centigrade.

[0040] As apparent from the results, the standard value of the Fotemperature i.e., ±0.5%, is satisfied when the thickness of the siliconerubber at the edge portion of the piezoelectric element is substantiallyequal to or larger than about 5 micrometers. If a silicone rubber in afluid state having a thixo index Ti that is larger than about 1.7 isused, it is easy to achieve a silicone rubber layer at the edge portionof a piezoelectric element having a thickness which is substantiallyequal to or larger than about 5 micrometers. TABLE 2 Thickness Fochanging ratio at each temperature (%) (micrometer) −20° C. 0° C. +20°C. +40° C. +60° C. +85° C. 0 0.28 0.18 0.00 −0.24 −0.68 −0.79 5 0.030.02 0.00 −0.05 −0.14 −0.16 8 −0.05 −0.02 0.00 −0.04 −0.18 −0.20 15−0.08 −0.03 0.00 −0.03 −0.11 −0.12

[0041] The present invention is not limited to the above describedpreferred embodiments. As a piezoelectric element, the present inventionis not limited to a piezoelectric trap component but may be apiezoelectric filter such as one disclosed in Unexamined Japanese PatentPublication No. 1-228310, a ceramic discriminator, or other suitableapparatus.

[0042] The number of terminals may be two or three.

[0043] It is possible to use the same silicone rubber for forming thefirst elastic material and the second elastic material.

[0044] As a method for providing the first elastic material, in additionto dropping by an iron or a dispenser, dipping the. edge portion of thepiezoelectric element into the first elastic material in a fluid stateis also possible. Further, as a method for providing the second elasticmaterial, other methods can be applicable in addition to the dippingmethod.

[0045] Further, a material other than silicone rubber is also applicablefor the elastic materials if such material has required characteristic.

[0046] While the invention has been particularly shown and describedwith reference to preferred embodiments thereof, it will be understoodby those skilled in the art that the forgoing and other changes in formand details may be made therein without departing from the spirit of theinvention.

What is claimed is:
 1. A piezoelectric component, comprising: apiezoelectric element; a first elastic material covering at least a pairof end portions of the piezoelectric element, the pair of end portionsincluding edge portions of the piezoelectric element; a second elasticmaterial covering the entire piezoelectric element and the first elasticmaterial; and an outer-cladding resin covering the entire circumferenceof the piezoelectric element covered by the second elastic material. 2.A piezoelectric component according to claim 1, wherein thepiezoelectric component is substantially rectangular shaped.
 3. Apiezoelectric component according to claim 1, wherein the thixotropicindex of the first elastic material in an unhardened state is largerthat that of the second elastic material in an unhardened state.
 4. Thepiezoelectric component according to claim 1, wherein the thixotropicindexes of the first elastic material and the second elastic material inan unhardened state are respectively larger than about 1.7.
 5. Thepiezoelectric component according to claim 1, wherein the first elasticmaterial and the second elastic material are the same.
 6. Thepiezoelectric component according to claim 1, wherein the first elasticmaterial and the second elastic material are different.
 7. Thepiezoelectric component according to claim 1, wherein the piezoelectriccomponent comprises a piezoelectric trap component.
 8. The piezoelectriccomponent according to claim 1, wherein the piezoelectric substrate ispolarized along the longitudinal direction of the major surfacesthereof.
 9. The piezoelectric component according to claim 1, furthercomprising divided electrodes on both end portions of a first of themajor surfaces of the piezoelectric substrate and a ground electrodedisposed at the approximate center portion of a second major surface ofthe piezoelectric substrate.
 10. The piezoelectric component accordingto claim 1, wherein the piezoelectric substrate vibrates in a thicknessshear vibration mode.
 11. The piezoelectric component according to claim1, wherein the hardness of the second silicone rubber is equal to orless than about 28 in Shore Hardness A.
 12. A method of manufacturing apiezoelectric component, comprising the steps of: forming an unhardenedfirst elastic material partially on at least a pair of end portions of apiezoelectric element, the pair of end portions including a edgeportions of the piezoelectric element; hardening the first elasticmaterial; forming an unhardened second elastic material on the entirecircumference of the piezoelectric element and the first elasticmaterial; hardening the second elastic material; and forming anunhardened outer-cladding resin on the entire circumference of thesecond elastic material covering the piezoelectric element and the firstelastic material; and hardening the outer-cladding resin.
 13. The methodaccording to claim 12, wherein the thixotropic index of the unhardenedfirst-elastic material is larger that that of the unhardened secondelastic material.
 14. The method according to claim 12, wherein thethixotropic indexes of the unhardened first elastic material and theunhardened second elastic material are respectively larger than about1.7.
 15. The method according to claim 13, wherein the thixotropicindexes of the unhardened first elastic material and the unhardenedsecond elastic material are respectively larger than about 1.7.
 16. Themethod according to claim 12, wherein at least one of the first elasticmaterial and the second elastic material is silicone rubber.
 17. Themethod according to claim 12, wherein the step of forming the firstelastic material is performed by one of dropping the first elasticmaterial by one of an iron and a dispenser and dipping the edge portionof the piezoelectric element into the first elastic material in a fluidstate.
 18. The method according to claim 12, wherein the step of formingthe second elastic material is performed by dipping the piezoelectricelement into the second elastic material in a fluid state.
 19. Themethod according to claim 12, wherein the step of forming theouter-cladding resin is performed by dipping the piezoelectric elementinto the outer-cladding resin in a fluid state.
 20. The method accordingto claim 12, wherein the total thickness of first and second layers atthe edge portions of the piezoelectric element is larger than thedifference between the maximum contraction amount of the outer-claddingresin and the maximum contraction amount of the piezoelectric element.